In a cellular communication system, a geographical region is divided into a number of cells served by base stations. The base stations are interconnected by a fixed network which can communicate data between the base stations. A mobile station is served via a radio communication link from the base station of the cell within which the mobile station is situated.
A typical cellular communication system extends coverage over an entire country and comprises hundreds or even thousands of cells supporting thousands or even millions of mobile stations. Communication from a mobile station to a base station is known as the uplink, and communication from a base station to a mobile station is known as the downlink.
The fixed network interconnecting the base stations is operable to route data between any two base stations, thereby enabling a mobile station in a cell to communicate with a mobile station in any other cell. In addition, the fixed network comprises gateway functions for interconnecting to external networks such as the Internet or the Public Switched Telephone Network (PSTN), thereby allowing mobile stations to communicate with landline telephones and other communication terminals connected by a landline. Furthermore, the fixed network comprises much of the functionality required for managing a conventional cellular communication network including functionality for routing data, admission control, resource allocation, subscriber billing, mobile station authentication etc.
The most ubiquitous cellular communication system is the 2nd generation communication system known as the Global System for Mobile communication (GSM). GSM uses a technology known as Time Division Multiple Access (TDMA) wherein user separation is achieved by dividing frequency carriers into 8 discrete time slots, which individually can be allocated to a user. Further description of the GSM TDMA communication system can be found in ‘The GSM System for Mobile Communications’ by Michel Mouly and Marie Bernadette Pautet, Bay Foreign Language Books, 1992, ISBN 2950719007.
Currently, 3rd generation systems are being rolled out to further enhance the communication services provided to mobile users. The most widely adopted 3rd generation communication systems are based on Code Division Multiple Access (CDMA) technology. Both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) techniques employ this CDMA technology. In CDMA systems, user separation is obtained by allocating different spreading and scrambling codes to different users on the same carrier frequency and in the same time intervals. In TDD, additional user separation is achieved by assigning different time slots to different users similarly to TDMA. However, in contrast to TDMA, TDD provides for the same carrier frequency to be used for both uplink and downlink transmissions. An example of a communication system using this principle is the Universal Mobile Telecommunication System (UMTS). Further description of CDMA and specifically of the Wideband CDMA (WCDMA) mode of UMTS can be found in ‘WCDMA for UMTS’, Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876.
In a 3rd generation cellular communication system, the communication network comprises a core network and a Radio Access Network (RAN). The core network is operable to route data from one part of the RAN to another, as well as interfacing with other communication systems. In addition, it performs many of the operation and management functions of a cellular communication system, such as billing. The RAN is operable to support wireless user equipment over a radio link of the air interface. The RAN comprises the base stations, which in UMTS are known as Node Bs, as well as Radio Network Controllers (RNC) which control the base stations and the communication over the air interface.
The RNC performs many of the control functions related to the air interface including radio resource management and routing of data to and from appropriate base stations. It further provides the interface between the RAN and the core network. An RNC and associated base stations are known as a Radio Network Subsystem (RNS).
3rd generation cellular communication systems have been specified to provide a large number of different services including efficient packet data services. For example, downlink packet data services are supported within the 3rd Generation Partnership Project 3GPP release 5 specifications in the form of the High Speed Downlink Packet Access (HSDPA) service.
In accordance with the 3GPP specifications, the HSDPA service may be used in both Frequency Division Duplex (FDD) mode and Time Division Duplex (TDD) mode.
HSDPA seeks to provide packet access services with a relatively low resource usage and with low latency.
Specifically, HSDPA uses a number of techniques in order to reduce the resource required to communicate data and to increase the capacity of the communication system. These techniques include Adaptive Coding and Modulation (ACM), retransmission with soft combining and fast scheduling performed at the base station.
In HSDPA, transmission code resources are shared amongst users according to their traffic needs. The base station (also known as the Node-B for UMTS) is responsible for allocating and distributing the HSDPA resources amongst the individual calls. In a UMTS system that supports HSDPA, some of the code allocation is performed by the RNC whereas other code allocation, or more specifically, scheduling is performed by the base station. Specifically, the RNC allocates a set of resources to each base station, which the base station can use exclusively for high speed packet services. The RNC furthermore controls the flow of data to and from the base stations. However, the base station is responsible for scheduling HS-DSCH transmissions to the mobile stations that are attached to it, for operating a retransmission scheme on the HS-DSCH channels, for controlling the coding and modulation for HS-DSCH transmissions to the mobile stations and for transmitting data packets to the mobile stations.
In HSDPA, each base station comprises a buffer for temporarily storing data to be transmitted to the mobile stations. Furthermore, HSDPA does not support soft handover and an HSDPA mobile station will at any given point be served by only one base station. When a hard handover occurs from one base station to another, a fixed time instant is determined for the handover. At this time instant, the mobile station switches from being supported by the first base station to being supported by the second base station. However, at the time of the handover, the buffers of the first base station may still comprise data for the mobile station which has not yet been transmitted. Accordingly, this data is merely discarded by the first base station, and retransmission techniques are used to recover the data via the second base station when the handover has taken place.
Although such process can be simple to implement, it tends to provide suboptimal performance. In particular, additional communication resource is used for the communication between the RNC and the two base stations as the discarded data must be transmitted from the RNC to the second base station. Furthermore, the retransmissions required to recover the discarded data results in a gap in the data stream received by the mobile station and can increase transmission delays substantially. For some services, such a gap may be noticeable to the user.
Hence, an improved system for radio link handover would be advantageous and in particular a system allowing increased flexibility, improved handover performance, an improved user experience, reduced resource requirements, reduced complexity and/or a reduced delay would be advantageous.